Pixel structure

ABSTRACT

A pixel structure suitable for being disposed on a substrate includes a thin film transistor (TFT), a first pixel electrode, a second pixel electrode, a scan line and a data line. The TFT disposed on the substrate includes a gate, a source, a first drain and a second drain. A main TFT is formed by the gate, the source and the first drain. A sub-thin film transistor (sub-TFT) is formed by the gate, the first drain and the second drain. The first pixel electrode is electrically connected to the first drain, and a portion of the first drain extends between the second pixel electrode and the substrate to form capacitor-coupling electrode. The second pixel electrode is electrically connected to the second drain of the sub-TFT. The scan line is disposed on the substrate and electrically connected to the gate, and the data line is electrically connected to the source.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of an application Ser. No. 11/840,995,filed on Aug. 19, 2007, now pending, which claims the priority benefitof Taiwan application serial no. 96120514, filed on Jun. 7, 2007. Theentirety of each of the above-mentioned patent applications is herebyincorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a pixel structure, and moreparticularly, to a pixel structure capable of improving display quality.

2. Description of Related Art

The rapid development of multi-media comes mostly as a result of therecent advance in the production of semiconductor devices and displayapparatus. Liquid crystal display, with its high display quality, goodspatial utilization, low power consumption and radiation-free operation,has gradually become the mainstream display product in the market. Toprovide better display quality to the liquid crystal display, all kindsof wide viewing angle liquid crystal displays have been developed. Themost common ones include, for example, the in-plane switching (IPS)liquid crystal display, the fringe field switching liquid crystaldisplay, the multi-domain vertical alignment (MVA) liquid crystaldisplay and so on.

FIG. 1 is a schematic cross-sectional view of a conventional pixelstructure. As shown in FIG. 1, the conventional pixel structure 100includes a substrate 112, a thin film transistor T, a capacitor-couplingelectrode 118 c, a first pixel electrode 119 a, a second pixel electrode119 b and an alignment film PI. The thin film transistor T mainlyincludes a gate 114, a gate insulating layer 116, a semiconductor layer117, a source 118 a and a drain 118 b. In FIG. 1, the thin filmtransistor T is a bottom gate structure, and the thin film transistor Tis covered by the passivation layer 120. More specifically, the thinfilm transistor T is disposed on the substrate 112, and thecapacitor-coupling electrode 118 c is electrically connected to thedrain 118 b of the thin film transistor T. In addition, the first pixelelectrode 119 a is electrically connected to the drain 118 b of the thinfilm transistor T, and the capacitor-coupling electrode 118 c is locatedbetween the second pixel electrode 119 b and the substrate 112.

In an ideal condition, the first pixel electrode 119 a is electricallyinsulated from the second pixel electrode 119 b, and the second pixelelectrode 119 b is coupled to the capacitor-coupling electrode 118 cunderneath. In other words, after turning on the active device T, thefirst pixel electrode 119 a and the second pixel electrode 119 b canhave different voltages so that the liquid crystals (not shown)corresponding to the first pixel electrode 119 a and the second pixelelectrode 119 b can have different inclining states. It should be notedthat residual charges on the second pixel electrode 119 b and thealignment film PI are difficult to remove because the second pixelelectrode 119 b is in a floating state. Consequently, the performance ofthe second pixel electrode 119 b will be affected by the residualcharges so that the problem of having a residual image in the nextdisplay picture needs to be solved.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a pixel structure capable ofpreventing the display of a residual image in a picture.

The invention also provides another pixel structure with good displayquality. The invention also provides a pixel structure suitable fordisposing on a substrate. The pixel structure of the invention includesa thin film transistor, a first pixel electrode, a second pixelelectrode, a scan line and a data line. The thin film transistor isdisposed on the substrate. The thin film transistor of the inventionincludes a gate, a source, a first drain and a second drain.Additionally, a main thin film transistor is formed by the gate, thesource and the first drain. A sub-thin film transistor (sub-TFT) isformed by the gate, the first drain and the second drain. When the mainthin film transistor and the sub-TFT are turned on, the conductingcurrent of the sub-TFT is substantially smaller than the conductingcurrent of the main thin film transistor, and their conducting currentsform a specific ratio. The first pixel electrode of the invention iselectrically connected to the first drain, and a portion of the firstdrain extends between the second pixel electrode and the substrate suchthat a capacitor-coupling electrode is formed. Moreover, the secondpixel electrode is electrically connected to the second drain of thesub-TFT. The scan line is disposed on the substrate and electricallyconnected to the gate, and the data line is electrically connected tothe source.

In an embodiment of the invention, the specific ratio is between0.05˜0.3, for example.

In an embodiment of the invention, the first pixel electrode has aplurality of slits.

In an embodiment of the invention, the second pixel electrode has aplurality of slits.

In an embodiment of the invention, the pixel structure further includesa common line distribution pattern disposed on the substrate andelectrically connected to a common voltage.

In an embodiment of the invention, at least a portion of the common linedistribution pattern extends along the edge of the first pixel electrodeand the second pixel electrode.

In the invention, a sub-thin film transistor having a very smallconducting current is electrically connected to a second pixel electrodecorresponding to the capacitor-coupling electrode. Therefore, the secondpixel electrode may utilize the sub-thin film transistor of theinvention to remove unwanted residual charges so as to prevent theperformance of the second pixel electrode from being adversely affectand effectively suppress the problem of having residual image in adisplay picture.

In order to make the aforementioned and other features and advantages ofthe invention comprehensible, embodiments accompanied with figures aredescribed in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic cross-sectional view of a conventional pixelstructure.

FIG. 2A is a pixel structure according to a first embodiment of theinvention.

FIG. 2B is a circuit diagram of the pixel structure according to thefirst embodiment of the invention.

FIG. 3A is a pixel structure according to a second embodiment of theinvention.

FIG. 3B is a circuit diagram of the pixel structure according to thesecond embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

First Embodiment

FIG. 2A is a pixel structure according to a first embodiment of theinvention.

FIG. 2B is a circuit diagram of the pixel structure according to thefirst embodiment of the invention. As shown in FIGS. 2A and 2B, thepixel structure 200 of the invention includes a thin film transistor220, a first pixel electrode 230 a, a second pixel electrode 230 b, ascan line 240 and a data line 250. The thin film transistor 220 isdisposed on a substrate 210 and electrically connected to the scan line240 and the data line 250. In practice, a switching signal can betransmitted through the scan line 240 to turn on the thin filmtransistor 220. After turning on the thin film transistor 220, a displaysignal can be transmitted to the first pixel electrode 230 a and thesecond pixel electrode 230 b through the data line 250.

The thin film transistor 220 of the invention includes a gate 222, asemiconductor layer 223, a source 224, a first drain 226 a, a seconddrain 226 b and a third drain 226 c. It should be mentioned that thethin film transistor 220 shown in FIG. 2A is a bottom gate structure.Obviously, the thin film transistor 220 can also be a top gatestructure. Here, the bottom gate structure is chosen only as an exampleand is not intended to limit the invention.

More specifically, the scan line 240 is electrically connected to thegate 222, and the data line 250 is electrically connected to the source224. The gate 222 can be part of the extension of the scan line 240, andthe source 224 can be part of the extension of the data line 250. Asshown in FIG. 2B, it should be mentioned that a thin film transistor 220a is formed by the gate 222, the source 224 and the first drain 226 a ofthe invention. A second thin film transistor 220 b is formed by the gate222, the source 224 and the second drain 226 b. A sub-thin filmtransistor (sub-TFT) 220 c is formed by the gate 222, the second drain226 b and the third drain 226 c. In particular, when the sub-TFT 220 cand the second thin film transistor 220 b are turned on, the conductingcurrent of the sub-TFT 220 c is substantially smaller than theconducting current of the second thin film transistor 220 b, and thereis a specific ratio between the conducting currents. For example, theconducting current of the sub-TFT 220 c can be 0.05 to 0.3 times theconducting current of the second thin film transistor 220 b.

It should be noted from FIGS. 2A and 2B that the first pixel electrode230 a of the invention could be electrically connected to the firstdrain 226 a of the first thin film transistor 220 a through a contactopening Cl. In addition, the second drain 226 b of the second thin filmtransistor extends to a portion between the second pixel electrode 230 band the substrate 210 such that a capacitor-coupling electrode 230 c isformed. In particular, the second pixel electrode 230 b can beelectrically connected to the third drain 226 c of the sub-TFT through acontact opening C2. In general, the first pixel electrode 230 a and thesecond pixel electrode 230 b may have a plurality of slits S so thatliquid crystals (not shown) can be divided into a plurality of domains.

Furthermore, the pixel structure 200 of the invention may also includesa common line pattern 242 disposed on the substrate 210. At least aportion of the common line pattern 242 of the invention extends alongthe edge of the first pixel electrode 230 a and the second pixelelectrode 230 b. Obviously, the common line pattern 242 shown in FIG. 2Acan have other shapes and can be selected on demand. Here, the shape ofthe common line pattern 242 is used only as an example and is notintended to limit the invention. In practice, the common line pattern242 only has to be electrically connected to a common voltage.

More specifically, after turning on the first thin film transistor 220a, the second thin film transistor 220 b and the sub-TFT 220 c, adisplay signal can be transmitted to the first pixel electrode 230 athrough the first thin film transistor 220 a. Here, it should bementioned that the first pixel electrode 230 a could form a firststorage capacitor Cst1 (as shown in FIG. 2B) with the common linepattern 242 underneath and form a first liquid crystal capacitor Clc1with a common electrode above an upper color filter (not shown).

Additionally, the second pixel electrode 230 b can couple with thecapacitor-coupling electrode 230 c underneath to form a couplingcapacitor Cc (as shown in FIG. 2B), and form a second storage capacitorCst2 with the common line pattern 242. On the other hand, a secondliquid crystal capacitor Clc2 can be formed by the second pixelelectrode 230 b and the common electrode of the upper color filter (notshown). In other words, the first pixel electrode 230 a and the secondpixel electrode 230 b can have different voltages. As a result, theliquid crystals (not shown) corresponding to the first pixel electrode230 a and the second pixel electrode 230 b can have different incliningstates so as to provide a wide viewing angle in the display.

It should be note that the performance of the second pixel electrode 230b would not be affected because the conducting current of the sub-TFT220 c is very small. Here, it should be mentioned that the residualcharges on the second pixel electrode 230 b of a previous displaypicture could be removed through the sub-TFT 220 c. As a result, theproblem of having residual charges on the second pixel electrode 119 b(as shown in FIG. 1) in the conventional design can be resolved.Therefore, the pixel structure 200 of the invention can effectivelysuppress residual image and improve display quality. Moreover, it shouldbe said the magnitude of the conducting current of the sub-TFT 220 c canbe suitably adjusted according to the actual requirements as long as itdoes not affect the normal performance of the second pixel electrode 230b.

Second Embodiment

FIG. 3A is a pixel structure according to a second embodiment of theinvention.

FIG. 3B is a circuit diagram of the pixel structure according to thesecond embodiment of the invention. As shown in FIGS. 3A and 3B, thepixel structure 300 of the invention includes a thin film transistor320, a first pixel electrode 330 a, a second pixel electrode 330 b, ascan line 340 and a data line 350. The thin film transistor 320 isdisposed on a substrate 310 and electrically connected to the scan line340 and the data line 350. In practice, a switching signal can betransmitted to turn on the thin film transistor 320 through the scanline 340. After turning on the thin film transistor 320, a displaysignal can be transmitted to the first pixel electrode 330 a and thesecond pixel electrode 330 b through the data line 350.

More specifically, the thin film transistor 320 of the inventionincludes a gate 322, a semiconductor layer 323, a source 324, a firstdrain 326 a and a second drain 326 b. It should be mentioned that thethin film transistor 320 shown in FIG. 3A is a bottom gate structure.Obviously, the thin film transistor 320 can also be a top gatestructure. Here, the bottom gate structure is chosen only as an exampleand is not intended to limit the invention.

In practice, the scan line 340 is electrically connected to the gate322, and the data line 350 is electrically connected to the source 324.The gate 322 can be part of the extension of the scan line 340, and thesource 324 can be part of the extension of the data line 350. Here, itshould be mentioned that the gate 322, the source 324 and the firstdrain 326 a of the invention form a main thin film transistor 320 a asshown in FIG. 3B. As shown in FIG. 3B, a sub-thin film transistor(sub-TFT) 320 b is formed by the gate 322, the first drain 326 a and thesecond drain 326 b. In particular, when the main thin film transistor320 and the sub-TFT 320 b are turned on, the conducting current of thesub-TFT 320 b is substantially smaller than the conducting current ofthe main thin film transistor 320 a, and there is a a specific ratiobetween the conducting currents. For example, the conducting current ofthe sub-TFT 320 b can be 0.05 to 0.3 times the conducting current of themain thin film transistor 320 a.

As shown in FIGS. 3A and 3B, the first pixel electrode 330 a of theinvention can be electrically connected to the first drain 326 a througha contact opening C3.

Moreover, part of the first drain 326 a extends into a portion betweenthe second pixel electrode 330 b and the substrate 310 such that acapacitor-coupling electrode 330 c is formed. In particular, the secondpixel electrode 330 b can be electrically connected to the second drain326 b of the sub-TFT 320 b through a contact opening C4. In general, thefirst pixel electrode 330 a and the second pixel electrode 330 b canhave a plurality of slits S so that liquid crystals (not shown) can bedivided into a plurality of domains.

Furthermore, the pixel structure 300 of the invention may also include acommon line pattern 342 disposed on the substrate 310. At least aportion of the common line pattern 342 of the invention extends alongthe edge of the first pixel electrode 330 a and the second pixelelectrode 330 b. Obviously, the common line pattern 342 shown in FIG. 3Acan have other shapes and can be selected on demand. Here, the shape ofthe common line pattern 342 is used only as an example and is notintended to limit the invention. In practice, the common line pattern342 only has to be electrically connected to a common voltage.

More specifically, after turning on the main thin film transistor 320 aand the sub-TFT 320 b, a display signal can be transmitted to the firstpixel electrode 330 a through the main thin film transistor 320 a. Here,it should be mentioned that a first storage capacitor Cst1 (as shown inFIG. 3B) could be formed by the first pixel electrode 330 a and thecommon line pattern 342 underneath. A first liquid crystal capacitorClc1 is formed by the first pixel electrode 330 a and a common electrodeof an upper color filter (not shown).

Additionally, a coupling capacitor Cc is formed by the second pixelelectrode 330 b and the capacitor-coupling electrode 330 c. Thecapacitor-coupling electrode 330 c underneath the second pixel electrode330 b is coupled to the second pixel electrode 330 b. A second storagecapacitor Cst2 is formed by the second pixel electrode 330 b and hecommon line pattern 342. On the other hand, a second liquid crystalcapacitor Clc2 is formed by the second pixel electrode 330 b and thecommon electrode above the upper color filter (not shown). In otherwords, the first pixel electrode 330 a and the second pixel electrode330 b can have different voltages. As a result, the liquid crystals (notshown) corresponding to the first pixel electrode 330 a and the secondpixel electrode 330 b can have different inclining states so as toprovide a wide viewing angle in the display.

It should be note that the performance of the second pixel electrode 330b would not be affected because the conducting current of the sub-TFT320 b is very small. Here, it should be mentioned that the residualcharges on the second pixel electrode 330 b of a previous displaypicture could be removed through the sub-TFT 320 b. As a result, theproblem of having residual charges on the second pixel electrode 119 b(as shown in FIG. 1) in the conventional design can be resolved.Therefore, the pixel structure 300 of the invention can effectivelysuppress residual image and improve display quality. Moreover, it shouldbe said the magnitude of the conducting current of the sub-TFT 320 b canbe suitably adjusted according to the actual requirements as long as itdoes not affect the normal performance of the second pixel electrode 330b.

In summary, a sub-thin film transistor having a very small conductingcurrent is electrically connected to a second pixel electrodecorresponding to the capacitor-coupling electrode. Therefore, the secondpixel electrode may utilize the sub-thin film transistor of theinvention to remove unwanted residual charges so as to prevent theperformance of the second pixel electrode from being adversely affectand effectively suppress the problem of having residual image in adisplay picture. Ultimately, the display quality is improved.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of theinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the invention covermodifications and variations of this invention provided they fall withinthe scope of the following claims and their equivalents.

1. A pixel structure for being disposed on a substrate, the pixel structure comprising: a thin film transistor, disposed on the substrate and comprising a gate, a source, a first drain and a second drain, wherein a main thin film transistor is formed by the gate, the source and the first drain and a sub-thin film transistor is formed by the gate, the first drain and the second drain, and when the main thin film transistor and the sub-thin film transistor are turned on, a conducting current of the sub-thin film transistor is smaller than a conducting current of the main thin film transistor, and the conducting currents form a specific ratio; a first pixel electrode, electrically connected to the first drain; a second pixel electrode, part of the first drain extends to a portion between the second pixel electrode and the substrate to form a capacitor-coupling electrode, wherein the second drain of the sub-thin film transistor is electrically connected to the second pixel electrode; a scan line, disposed on the substrate and electrically connected to the gate; and a data line, electrically connected to the source.
 2. The pixel structure according to claim 1, wherein the specific ratio is between 0.05˜0.3.
 3. The pixel structure according to claim 1, wherein the first pixel electrode has a plurality of slits.
 4. The pixel structure according to claim 1, wherein the second pixel electrode has a plurality of slits.
 5. The pixel structure according to claim 1, further comprising a common line pattern disposed on the substrate and electrically connected to a common voltage.
 6. The pixel structure according to claim 5, wherein at least part of the common line pattern extends along an edge of the first pixel electrode and the second pixel electrode. 